The present invention relates to an image processing apparatus for use with digital photocopying machines and similar machines utilizing a laser beam.
As for image processing apparatuses, for example, a digital photocopying machine utilizing a laser beam, as shown in FIG. 9, often comprises a scanner section 300, an image processor section 400, and a printer section 100.
The scanner section 300 is run to optically scan on a manuscript 200. Image information of the manuscript then can be converted to an optical image. The optical image is fed to the image processor section 400 to be converted to a video signal, which is subjected to necessary video signal processes.
The video signal processes include an enlargement or a reduction process, a shading process, a window process, and so forth.
For color images, they include an additional color ghost correction process.
The printer section 100 can record the image on the basis of a digital video signal, or a video data, of specific bits formed by the image processor section 400.
FIG. 10 shows an example of the printer section 100. In this example, an electrophotographic printer utilizing a photosensitive drum is used, and a laser beam is used as light source for forming an electrostatic latent image.
In FIG. 10, a video data DATA output of the image processor section 400 is fed to a modulation circuit 110. The modulation circuit 110 can form a pulse-width modulated (PWM) signal SPWM on the basis of the video data DATA.
The modulated signal SPWM formed by the modulation circuit 110 is fed to a semiconductor laser 931 through a laser driver circuit 932. The modulated signal modulates the laser beam internally. The laser driver circuit 932 can be controlled by a control signal led from a timing circuit 933 so that a drive state is allowed only for a horizontal and a vertical effective section.
To the laser driver circuit 932 is fed back a signal indicating an energy amount of the laser beam from the semiconductor laser 931. Driving of the semiconductor laser 931 is controlled so that the amount of energy of the laser beam can be made constant.
The laser beam output of the semiconductor laser 931 is led to a polygonal mirror 935 to be deflected. The laser beam deflected by the polygonal mirror 935 is detected at its scanning start point by an index sensor 936, and is converted to a voltage signal through an I/V amplifier 937 to form an index signal SI. The index signal SI (not shown) is fed to a control means that can control an optical scanning timing and the like of the scanner section 300.
A number 934 indicates a motor driver circuit for turning the polygonal mirror 935. Its turn on-off signal is fed from the timing circuit 933.
FIG. 11 shows an example of an image exposing system, or a laser beam scanner on which the laser beam is focused.
The laser beam coming out of the semiconductor laser 931 impinges the above-mentioned polygonal mirror 935 by way of mirrors 942 and 943. The laser beam is deflected by the polygonal mirror 935, and is irradiated to a surface of the photosensitive drum 130 through a focusing f- F lens 944 to focus to a specific diameter. Elements 945 and 946 are cylindrical lenses for correcting an inclination.
The laser beam is made to scan a surface of the photosensitive drum 130 in a specific direction a by the polygonal mirror 935 to produce exposure for formation of the electrostatic latent image corresponding to the video data.
Though not shown, toner of an opposite charge is attracted to the electrostatic latent image to develop an image. A sheet of recording paper is put over the toner image. A corona charger can give a charge opposite to that of the toner to the recording paper to transfer the toner image to the recording paper. The transferred toner image, further, is fixed onto the recording paper by way of heat or pressure.
FIG. 12 is an example of a modulation circuit provided in the printer section 100. In the figure, a dot clock DCK synchronized with a video data DATA is fed to through a buffer 21 to an integrator 22, comprising a variable resistor 22a and a capacitor 22b. A signal output of the integrator 22 is fed through a serial circuit, comprising a resistor 23, a buffer 24, and direct-current blocking capacitor 25, to a comparator 26 as a pattern signal Sp.
The pattern signal Sp can be adjusted in amplitude thereof by the variable resistor 22a so that the entire pattern signal Sp can be just put in a full scale of OOH to FFH of a digital-to-analog converter 28 which will be described later. It also can be adjusted in an offset value of direct current thereof by a variable resistor 27.
A video data DATA of 8 bits, for example, is fed to the digital-to-analog converter 28, in which it is converted to an analog signal. The analog signal is fed to a comparator 26 as a video signal Sv. A symbol CLK in the figure is a clock for digital-to-analog conversion.
The comparator 26 can compare the pattern signal Sp from the integrator 22 with the video signal Sv from the digital-to-analog converter 28. The comparator 26 outputs a pulse-width modulated signal SPWM on the basis of the video data DATA is fed to a NAND circuit which constitutes the comparator 26.
In the above-mentioned circuit configuration, if the dot clock DCK has a waveform shown in FIG. 13A, a triangle pattern signal Sp indicated by a solid line in FIG. 13B is fed to the comparator 26. If the video signal Sv is a waveform indicated by a broken line as shown in FIG. 13B, therefore, the comparator 26 can output the pulse-width modulated signal SPWM as shown in FIG. 13C.
However, the dot clock DCK needed to form the mentioned pattern signal Sp will have adversely produced a large distortion in its waveform due to standing waves, external noises, and the like during transmission. This causes the modulated signal SPWM not to be exactly formed, resulting in deterioration of a reproduced image.
For example, if the waveform of the dot clock DCK is normal as shown in FIG. 14A, the pattern signal Sp obtained will be correct as shown in FIG. 15A. If a duty cycle of the dot clock DCK changes as shown in FIG. 14B, on the other hand, the pattern signal Sp will change in its amplitude as shown in FIG. 15B because its rise time and fall time are not periodical. Also, if a noise is mixed with the dot clock DCK as shown in FIG. 14C, the pattern signal Sp will have noise mixed therein as shown in FIG. 15C. Further, if the dot clock DCK changes in its amplitude as shown in FIG. 14D, the pattern signal Sp will change in its amplitude as shown in FIG. 15D because its rise and fall inclinations cannot be identical.
In order to overcome the resulting distortion of the pattern signal Sp, it can be considered that the dot clock DCK should be frequency-divided to be free of the duty cycle change and the like. However, in that way, the pulse-width modulation cannot be made in units of dot clock, resulting in deterioration of the resolution of the reproduced image.
It should be noted that a solid line of FIG. 13D indicates the pattern signal Sp formed by frequency-halving the dot clock DCK. The modulated signal SPWM is the one shown in FIG. 13E wherein the pulse-width modulation is made in units of two dot clocks.
In view of the foregoing, it is a general object of the present invention to provide an image processing apparatus that can reproduce a quality image without the above-mentioned defects.